1. Field of the Invention
This invention relates to control actuators for aircraft, particularly helicopters, and, more particularly, to improvements in a series, trim, artificial feel actuators and combinations thereof of the general character disclosed in copending application Ser. No. 912,165, filed June 2, 1978 in the name of Carl D. Griffith and entitled "Aircraft Servoactuator Apparatus" now U.S. Pat. No. 4,228,386, said application also being assigned to the same assignee as the present invention.
2. Description of the Prior Art
Heretofore, the installation of an automatic Stability Augmentation System (SAS) and/or an Automatic Flight Control System (AFCS) as original or retrofit aircraft equipment, particularly in helicopters, normally comprised a number of separated components usually located at separate and convenient points in the control rigging (illustrated schematically in FIG. 1 of the accompanying drawings). Such components included a series actuator, usually contained within a control rod for increasing or decreasing its length in accordance with an electrical command from the SAS or AFCS computer, automatic and manual control authority limit stops, stick position transducers, an artificial feel spring mechanism having detent switches and a manual and/or automatic trim actuator and brake mechanism, and the like. In the retrofit of such actuators and associated components particularly in helicopters, the installation was more difficult, involving costly and time consuming modifications to the existing manual flight controls and linkage systems. As the above items are typically required for each axis of control, the SAS and/or AFCS original installation and retrofit becomes extremely difficult and time consuming adding not only to the aircraft's overall weight but also, significantly, to its total cost of ownership, considering maintenance and replacement costs.
Aircraft servoactuators are generally of two types: parallel and series, both of which are well known in the aircraft automatic control art. The parallel actuator is hard-mounted or bolted directly to the airframe and thus not severely affected by a high vibration environment as compared with conventional series actuator installations. A parallel actuator system is normally the autopilot actuator wherein its movement is reflected both at the control surface and the control stick. A series actuator, however, is not normally secured to the aircraft body structure but, rather is coupled in the control linkages or push-rod assemblies between the pilot's stick and the control surface; that is, it is floating or ungrounded and hence severely adversely affected in the high vibrating environment usually associated with a helicopter. The series servo motion does not move the pilot's stick and normally has a limited control authority over the control surface. Furthermore, as stated, the series actuator is usually installed in the control pushrods which, particularly in a retrofit situation, alters their natural resonances to local vibrations such as produced by the sustaining rotor.